Unable to keep belt centered - Sole F80

Hello.
Been hapily using my treadmill for about 4 years without any problems.
I had to replace a cracked deck board, thus had to loosen the belt to replace it, and since then i have been unable to keep the belt centered.
I can literally spend hours micro-adjusting the tension bolts and no matter what i do, it continues to move one side or the other.
3 mph with nobody on it - can keep it centered
3 mph with me on it - starts to move
5 mph with me on it - can keep it centered
5 mph without me on it - starts to move
7 mph with me on it - can keep it centered
10mph with me on it - starts to move

I could go on and on and on

When i say it moves, i mean lucky to get one minute before it slowely start to shift....and of course it moves very slowely but slow enough to fully get to one side after 5 mins max

From completely slack, i usually do 10 clockwise turns of the bolt....and that is what Sole told me to do.
The seam on the belt that runs diagonally across seems absolutely fine so i dont think the belt needs changing but i cant think what else it could be

Driving absolutely mential